A recently published paper provides new details on how some relatives of the …

Since the early 1980s, the story of how whales walked into the sea has become one of the most celebrated of all evolutionary transitions. Pakicetus, Ambulocetus, Rodhocetus, and many, many more—these fossil whales with legs have beautifully demonstrated how land-dwelling mammals became adapted to life at sea. But between 50 million and 40 million years or so ago, whales were just going through a transition that many other vertebrate groups had gone through before. They were not the first vertebrates to return to the sea, nor were they the last, and a paper recently published in Paleobiology by paleontologists Johan Lindgren, Michael Polcyn, and Bruce Young has traced the history of how a very different group of animals got their sea legs.

Mosasaurs were formidable oceanic predators. Take a Komodo Dragon, put flippers on it, and, in some cases, blow it up until it’s over 40 feet long and you’ll have some idea of what these Cretaceous marine lizards were like. Their fossil record—stretching over 27 million years—is also relatively well known, and so the mosasaurs provided Lindgren and colleagues with a good opportunity to see how these peculiar animals evolved.

The first thing you need to know about mosasaur evolution is that the way they swam was constrained by their anatomy. Whales provide a good counterpoint. The ancestors of whales were wolf-like animals which carried their limbs under their bodies, and when they walked, their spines undulated in a vertical plane. That’s why whales swim by beating their tails up and down—their mode of swimming is the product of an anatomical precondition from when their ancestors dwelt on land. The ancestors of mosasaurs, on the other flipper, moved like lizards—that is, their spines were more flexible from side to side. It’s no wonder, then, that mosasaurs swam by beating their tails back and forth, just like fish and that other group of famous marine reptiles, the ichthyosaurs.

So mosasaurs were side-to-side swimmers, and one of the genera taken to represent the early stage of their evolution is Dallasaurus. This was not a gigantic sea monster. Dallasaurus was small—less than three feet long—and it did not have the highly modified tail and flippers of the later, open-ocean mosasaurs. For example, the upper arm elements of Dallasaurus were relatively long, preserving a more archaic anatomical construction, than the shortened upper arm elements which helped keep the flippers stable for their roles as rudders in later mosasaurs. (Similar modifications of the upper arm can be seen in whales, too. The mechanics of swimming provided the selective pressure for parts of these very difference animals to be adapted in similar ways.)

A handful of recent discoveries have helped paleontologists better understand just how much some of the later mosasaurs became modified to life at sea. Mosasaurs have traditionally been reconstructed with long, thin, lizard-like tails. They did not appear to have any specialized tail fins as seen in the shark-like ichthyosaurs. Yet evidence that some mosasaurs had such structures has now been found. Skeletons of Plotosaurus and Platecarpus appear to exhibit downward kinks in the posterior part of the tail which could have supported fleshy tail fins. (Significantly, the part of the tail which supports the tail fin kinks upwards in sharks but downwards in marine reptiles—perhaps as a result of some kind of constraint or contingency.) These mosasaurs may be another case—with ichthyosaurs and crocodiles—of marine reptiles evolving prominent tail fins independently.

Lindgren and co-authors only looked at four representative mosasaur genera—Dallasaurus, Clidastes, Mosasaurus, and Plotosaurus—but together these creatures cover almost the entire span of mosasaur history and provide a rough idea of how the lizards changed through the Cretaceous. As might be expected, earlier mosasaurs lived nearer to shore in shallow environments, whereas later, more specialized forms—such as Plotosaurus—were open ocean cruisers which have been found in deposits indicating deeper environments. The rough picture is similar to that seen among fossil whales—easing into coastal environments and only later spreading far and wide. The same is true of the way the vertebrae of mosasaurs became evolutionarily modified for swimming. In early mosasaurs, the tail vertebrae were more or less the same and unspecialized. By the time of Mosasaurus and Plotosaurus, however, the tail had become divvied up into several different functional regions which enhanced swimming ability.

There is far more detail in the paper, of course—the entire thing runs 25 pages—but what strikes me is how very different vertebrates, even those with different anatomical constrains, eased into the seas in similar ways. Early whales were up-and-down swimmers, while mosasaurs were side-to-side swimmers, yet they both started off in the shallows and underwent a sequence of modification in which their tails became specialized into specific modules suited for swimming. This is wonderful stuff—when contingency, constraint, and convergence meet together in a great transformation.

Top Image: A modern restoration of the mosasaur Platecarpus by Dmitry Bogdanov. Image from Wikipedia.

This article needs some serious editing - it'd full of mistakes. The blurb on the front page refers to mosasaurs as ancestors of whales - clearly written by someone who didn't read the article. And later int the article it refers to whale ancestors as wolf-like. Not so much. Whales are artiodactyls, like antelopes etc. Their closest living relative is the hippopotamus. Other posters have noted other mistakes already. Please fix this article, and don't post these things without checking them out. It's not like there is time pressure on this one!

And later int the article it refers to whale ancestors as wolf-like. Not so much. Whales are artiodactyls, like antelopes etc. Their closest living relative is the hippopotamus.

The extent to which whale ancestors could be described as wolf-like is actually under some debate. Pakicetus is apparently a carnivore, and has been reconstructed as both a "terrestrial mammal, no more amphibious than a tapir", and as a semi-aquatic crocodile-like mammal.

The point was, however, that the plane of spinal motion for the proto-whales was vertical, while the plane of spinal motion for the proto-mosasaurs was horizontal. Let's not get overwhelmed by the peripheral details.

brionl wrote:

Quote:

Top Image: A modern restoration of the mosasaur Platecarpus by Dmitry Bogdanov. Image from Wikipedia.

My understanding is that the closest modern relative of the mosasaur is the Komodo dragon, and both evolved from a terrestrial ancestor.

According to M. Lee in The phylogeny of varanoid lizards and the affinities of snakes (1997), this isn't so. While mosasaurs, evolutionary speaking, are closely related to varanoids, their closest living relatives are, perhaps a bit surprising, the snakes. While the mentioned paper does contain some rather far-reaching conclusions, the phylogenetic tree constructed is mostly accepted among paleontologists.

The mechanics of swimming provided the selective pressure for parts of these very difference animals to be adapted in similar ways.)

You mean 'different' animals?

Other than that, I find this stuff to be awesome. I was recently able to go through the caves of St. Pietersburg in Maastricht, Netherlands. A lot of the tour talked about these giant sea creatures and I rather dismissed it (was looking at other stuff), but it's great to get a read into what really was going on.

It's still hard to wrap my head around the timescales on evolution. Pokemon has forever scarred me in that regard >.<

The ancestors of whales didn't become carnivores until they had already taken to living in aquatic environments (or at least semi-aquatic). The wolf comparison is about as opposite as I can imagine the whale's terrestrial ancestors to be: small deer-like creatures.

polyfemos wrote:

Dr. Jay wrote:

My understanding is that the closest modern relative of the mosasaur is the Komodo dragon, and both evolved from a terrestrial ancestor.

According to M. Lee in The phylogeny of varanoid lizards and the affinities of snakes (1997), this isn't so. While mosasaurs, evolutionary speaking, are closely related to varanoids, their closest living relatives are, perhaps a bit surprising, the snakes. While the mentioned paper does contain some rather far-reaching conclusions, the phylogenetic tree constructed is mostly accepted among paleontologists.

Also, the paper in itself is a work of art, read it :)

I'm with you on snakes being the best candidate for living relative. Mosasaurs had double-hinged jaws if I remember correctly, a trait shared only by snakes.

I did learn a lot from this article though, I've always wondered why whales had a different body-plan than previous marine vertebrates in regards to locomotion.

Lizards being cold-blooded, I'd presume there was an early adaption to allow them to survive in water. Is there any way of working out which of these fossils were cold-blooded and which were hot-blooded?

Take a Komodo Dragon, put flippers on it, and, in some cases, blow it up until it’s over 40 feet long and you’ll have some idea of what these Cretaceous marine lizards were like

Lizards? Seriously? While we believe mosasaurs to have been squamates, "lizards" is a bit of a push.

Um. Except that does make them lizards (which would be best applied to the node uniting rhynchocephalia and squamata, and all descendants). Snakes are lizards too, so the tentative phylogenies that alternate between their nearest ancestor being snakes and varanoids does not affect this conclusion.

I'm with you on snakes being the best candidate for living relative. Mosasaurs had double-hinged jaws if I remember correctly, a trait shared only by snakes.

Correct, snakes and mosasaurs shares a few derived characters in the cranial skeleton. This is fortunate, for biologists at least, since snakes doesn't have much of a post-cranial skeleton, and the cranial skeleton is heavily derived compared with other squamates. Mosasaurs, having a post-cranial skeleton (shoulder-girdles being critical) therefore helps place snake in the phylogenetic tree.

Sir_Anonymous wrote:

I did learn a lot from this article though, I've always wondered why whales had a different body-plan than previous marine vertebrates in regards to locomotion.

While I believe the explanation given in the article is the correct one, the movement of whales isn't without it's problems. While we know for sure how modern cetaceans swim, it doesn't make sense for basilosaurs (but then, not much makes sense for basilosaurs).

Lizards being cold-blooded, I'd presume there was an early adaption to allow them to survive in water. Is there any way of working out which of these fossils were cold-blooded and which were hot-blooded?

I am not sure if all "lizards" throughout history have been cold blooded, so the fact that they are lizards may not necessarily mean that they were cold-blooded. Also I don't think that being cold-blooded necessarily causes an issue with surviving in the water. Aren't most fish and sharks cold-blooded?

More to the point there are a handful of ways that scientists can estimate body temperature solely from skeletons. The most recent and most accurate method is through an examination of the makeup of the teeth. Apparently the molecular structure of teeth changes subtly based on temperature, and the materials within the teeth that do this are present in teeth across a very wide range of animals. An article discussing this method can be found here: http://content.usatoday.com/communities ... eratures/1

I did learn a lot from this article though, I've always wondered why whales had a different body-plan than previous marine vertebrates in regards to locomotion.

Carl Zimmer has a really great book about both the emergence of vertebrates onto the land and the return of the mammals to the sea as whales. It's called At the Water's Edge, and you should check it out if you haven't already.

wintersnight wrote:

Lizards being cold-blooded, I'd presume there was an early adaption to allow them to survive in water. Is there any way of working out which of these fossils were cold-blooded and which were hot-blooded?

You know, fish and sharks are also "cold-blooded" (well, mostly). It's not unusual for large, active marine predators. In fact whales, dolphins, seals, penguins etc. are pretty exceptional in being warm-blooded ones. That said, it's possible that other marine reptiles were warm-blooded; I think there's evidence for it in ichthyosaurs and plesiosaurs. Mosasaurs were more like "ambush" predators, though, while the other two were always active and probably needed a constant body temperature to keep up their cruising. That wouldn't be unlike swordfish and some sharks. Certain species can somewhat regulate their body temperature (but not enough to be "warm-blooded" like mammals and birds) and also mostly hunt by constantly moving.